But the president suspends his drive for a military strike to explore Russia's proposal that Syria turn over its chemical weapons.

WASHINGTON — President Obama suspended his drive to win congressional authorization for punitive military strikes against Syria on Tuesday, as his administration and U.S. allies began the diplomatic haggling with Russia that could lead to a peaceful resolution of the crisis.

In a rare prime-time address from the White House, Obama declared that he saw "encouraging signs" in negotiations sparked by an unexpected Russian proposal to place Syria's chemical weapons stockpiles under international control. But the president also counseled caution, and argued that the U.S. must maintain the threat of an attack to put pressure on the Syrian government.

In vivid language, Obama charged that President Bashar Assad's government "gassed to death" more than a thousand people in Damascus suburbs on Aug. 21.

"What kind of world will we live in if the United States of America sees a dictator brazenly violate international law with poison gas and we choose to look the other way?" Obama said in remarks delivered from the East Room. "Our ideals and principles, as well as our national security, are at stake in Syria, along with our leadership of a world where we seek to ensure that the worst weapons will never be used."

Obama's 16-minute speech presented a dual challenge for the president: building public support for his decision to launch military strikes while also explaining his eleventh-hour decision to shift gears.

Diplomats in Paris, Damascus, Moscow and Washington worked through the details of the Russian proposal — running into early signs of difficulty. Secretary of State John F. Kerry plans to travel to Geneva on Thursday to meet with his Russian counterpart.

"It's too early to tell whether this offer will succeed," Obama said in his speech, "and any agreement must verify that the Assad regime keeps its commitments. But this initiative has the potential to remove the threat of chemical weapons without the use of force."

In meetings earlier in the day with senators on Capitol Hill, Obama asked for time to sort through the options. Senate leaders, in a sign of the deep reluctance to endorse the president's push for another military intervention, readily complied.

Obama's speech originally had been intended as the keynote of a week of meetings, briefings, speeches and phone calls aimed at lawmakers whose support Obama would need to win a vote on the use of force against Syria. But the Russian proposal dramatically changed the context.

The speech was rewritten until late in the day Tuesday as the president and his aides wrestled with how to present a case for going forward even as they were asking Congress to pause.

With polls showing Americans opposed to a strike by roughly a 2-1 ratio, the White House sought to persuade lawmakers to buck public opinion and back Obama on a vote some say could determine the future of his presidency and U.S. credibility abroad.

In his remarks Tuesday, Obama acknowledged that a war-weary public was unlikely to support another conflict. But he argued it was a moral responsibility and matter of national security. The president stressed that unlike the wars in Iraq and Afghanistan, military action would be limited — but answered lawmakers who have wondered if a "pinprick" strike would make any difference.

"Let me make something clear: The United States military doesn't do pinpricks," Obama said. "Even a limited strike will send a message to Assad that no other nation can deliver."

In a joint statement after the speech, Sens. John McCain (R-Ariz.) and Lindsey Graham (R-S.C.), two of the most prominent backers of a military strike, said they "regret that he did not lay out a clearer plan to test the seriousness of the Russian and Syrian proposal."

Freshman Rep. Ann McLane Kuster (D-N.H.), an enthusiastic backer of Obama in the 2008 primaries, said she continued "to have very grave concerns about the unintended consequences of U.S. military intervention."

Congressional resistance to entering another conflict seemed to firm up with the sudden appearance of the Russian proposal.

Senate Majority Leader Harry Reid (D-Nev.) indefinitely postponed an initial vote on an authorization resolution approved by the Foreign Relations Committee last week. He said he would be satisfied with a diplomatic solution. "I'm not a blood-and-thunder guy. I'm not for shock and awe," he said.

Meanwhile, key senators are sketching the outline of an amended resolution. It would require the United Nations to pass a resolution to remove all chemical weapons in Syria by a defined date. If that failed, the use of force would be authorized, according to a source familiar with the talks.